化工专业英语课文21

Lesson 21 The Water We Drink

Despite the abundance of rivers, streams, and lakes throughout most of the populated land area, chances are that the water coming from your kitchen faucet or from a public water fountain was drawn up from the reservoir of ground water. Reaching the surface through wells or springs, this source provides the drinking water for about half the people in the United States and about three-quarters of those of our rivers, streams, and lakes. In chemical terms, ground water and the water we draw from our rivers and other bodies of water are solutions of solutes in a solvent. Water is the solvent; the substances the water picks up in its travels from the clouds, through the earth and into our faucets are the solutes. It’s a combination of the chemistry of the solutes and their concentrations that determines whether the water is polluted.

Some of the solutes have been in earth’s water since rain began to fall on the newly formed planet. Partly because of rainwater’s normal acidity (resulting from the carbonic acid it contains) and partly because water itself is a very good solvent for many substances, the rainfall that passes through the soil picks up a variety of minerals from the earth itself. As a result, all the waters of the earth, including those that feed our public and private water supplies and those that furnish commercially bottled “natural” or “mineral” drinking water, contain a variety of minerals in a range of concentrations. Among these are calcium, iron, magnesium,

potassium, and sodium cations and fluoride anions. The concentrations of representative minerals found in natural spring water of the French Apls. Minerals such as these are usually harmless or beneficial to most of us at their typical levels in natural and in commercially bottled drinking water. And at even higher levels as well. Some of us, for example, take mineral supplements that provide calcium, iron, or magnesium at levels far higher than those found in drinking water.

These are hazards, though. A few of the chemicals that can enter our water supplies are particularly toxic even at what may seem to be very low concentrations. Contamination by these substances comes as rainwater picks up residues of agricultural fertilizers and pesticides, known as agricultural runoff, and from industrial, urban, and household wastes dumped onto the surface or injected just below it. Any of these can be carried by the natural flow of rainwater into groundwater.

To ensure the safety and high quality of public drinking water, the U.S. Congress passed the Safe Drinking Water Act of 1974, which establishes, among other things, maximum drinking water levels for specific, potentially hazardous chemical contaminants. Some of the minerals controlled by the Act, while representative organic compounds. When present in concentrations, the minerals and the organic substances exceed the safety standards set by the Safe Drinking Water Act.

Curiously, the Act also applies to some kinds of commercially

bottled water, but not to others. The federal standards for tap water apply as well to “bulk” or “commodity” water, the kind that comes in large jugs and can be dispensed through water coolers. “Mineral water”, which we’d expect to be high in minerals anyway, is exempt from the standards of the Safe Drinking Water Act, as are both seltzer and club soda. These last two are considered soft drinks and are covered by regulations of the Food and Drug Administration. In 1990 the sparking drink Perrier was found to contain benzene at a concentration of 15 parts per billion, triple the FDA’s permitted maximum of 5 parts per billion (ppb). The company withdrew 72 million bottles of the drink from store shelves and restaurants while it located and corrected the source of contamination.

Of course, not all water pollution comes from accidents or the careless or deliberate dumping of eater chemicals or from agricultural runoff. Some results simply from the way our society operates. Just as the high concentration of cars in and near our cities, for example, contributes to air pollution, the large number of gasoline stations that serve these cars are potential sources of pollution. Old and rusting underground tanks in these stations (some of which have been left abandoned in the ground as the stations closed) leak gasoline, which eventually travels into the groundwater. Whether this presents a major threat to our drinking water is still uncertain, but it does illustrate one of the many less visible sources of contamination.